1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly relates to an active matrix liquid crystal display device driven by a transverse electric field driving method.
This application is based on Patent Application No. Hei 10-173182 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
An active matrix-type liquid crystal display device (hereinafter, abbreviated as AMLCD), which uses thin film field effect-type transistors as switching elements of the pixels, is widely used, due to its high image quality, as a display device for portable computers or as monitors for small size desk-top computers.
Recently, a method of display using a transverse electric field has been proposed for improving the angle-of-visibility characteristics for the purpose of realizing a higher image quality. This method is disclosed in Japanese Patent Application, First Publication No. Hei 5-505247, which controls the quantity of transmitted light by changing the direction of the anisotropic axis (director) of the liquid crystal by applying a voltage between the pixel electrodes and counter electrodes formed in parallel to each other on the first substrate on which TFTs (thin film transistors) are formed.
In this method of driving the liquid crystal display device, since the director rotates on the substrate surface when the voltage is applied between the pixel electrode and the counter electrode, good images are obtained over a wide viewing angle, in contrast to the twisted nematic method, wherein, the problem arises that due to rising of the director from the substrate, the relationship between the quantity of the transmitted light and the applied voltage shows a large change depending upon the viewing angle such as a viewing angle from a direction of the director or a viewing angle from the direction normal to the substrate.
The method shown in FIG. 5 is widely used, since it is possible to maintain the black color at a reduced brightness, in which method the liquid crystal layer is arranged in a homogeneous orientation, and is inserted between two polarizing plates, the angles of polarization of which are orthogonal to each other, and one of the polarization angles is made identical with the orientation of the liquid crystal, and the display color is made black when no voltage is applied, and the display color turns white, by twisting the liquid crystal into the direction of the electric field when applying a voltage.
In this liquid crystal driving mode, since the electric field is applied in the transverse direction, there is no transparent electrode at the liquid crystal side of the second substrate which faces the first substrate on which TFTs are formed. Therefore, color layers of the color filters on the second substrate (hereinafter, abbreviated as CF substrate), and a black matrix (hereinafter, abbreviated as BM) are not shielded electrically. Accordingly, the charge distribution inside the color filter is changed by the voltage applied to the liquid crystal, whereby the electric field is disturbed. Such disturbance causes cross-talk and after images, which results in deteriorating the image quality of the display device. Particularly, when the BM potential differs from the potential of the counter electrode, the electric charge near the BM migrates to the display opening portion, which causes after images.
A measure to prevent this electric charge migration is proposed in Japanese Patent Application, First Publication No. Hei 9-269504, in which a BM made of a conductive material is fixed at the same potential as that of the counter electrode by connecting the BM to the TFT substrate.
Although a method of increasing the resistance of the BM is proposed, to obtain a panel without having uneven indications, in Japanese Patent Application, First Publication No. Hei 9-43589, the method is not sufficient to solve the above problem by itself.
Furthermore, another method is proposed for a TN-type liquid crystal panel to float the transparent electrode formed, facing the liquid crystal, on the second substrate, in Japanese Patent Application, First Publication No. Hei 9-263021. However, this method is not a direct solution of the above problem, because it is related to a liquid crystal panel driven by applying an electric field in the transverse direction and floating is applied to the BM, which is not directly related to the formation of the electric field of the display opening.
Another method is proposed in the Domestic Re-publication of PCT International Publication No. WO95/25291, in which a sealed electrode is formed on the second substrate, and a fixed potential is applied to the sealed electrode to give the sealed electrode the role of the BM. In addition to the above structure, a method is proposed to provide a slit in the sealed electrode in order to prevent a delay in the signal caused by capacitive coupling between the sealed electrode at a fixed potential and signal lines. However, the above method does not constitute a substantial solution to the above problem, since the above method uses a BM in a floating condition and, in order to provide an area which overlaps with electrodes on the active matrix substrate for the purpose of bringing the potential of the black matrix near to that of the counter electrode, the slit is formed only on a part of the black matrix, which overlaps with the scanning lines.
In general, in the liquid crystal display device using a transverse electric field, an overcoat layer is formed on the color layer to prevent the diffusion of impurities from the color layer to the liquid crystal layer. In order to supply a potential from the TFT substrate to the BM, it is necessary to expose the BM by removing the overcoat layer around peripheral area, which causes the problem that an increased number of production processes is required. Additionally, it is necessary to reduce the resistance of the BM in order to suppress fluctuation of the BM potential caused by fluctuations of the signal line potentials, since the BM is capacitively coupled with many signal lines, so that the problem arises that the range of materials for forming the BM is restricted when the BM is applied to a large area.
Thus, a method to fix the potential of the BM is not advantageous, from the point of view of manufacturing an active matrix-type liquid crystal display panel at a reduced cost. However, as described above, if the BM is floated, the BM potential fluctuates as a result of the potential fluctuation on the TFT substrate, which seriously affects the image quality, irrespective of the resistance of the BM.
Immediately after starting display of the liquid crystal panel, the potential of each point on the BM coincides with that of the electrodes which overlap on the TFT substrate. That is, the potential of the BM adjacent to an area, where the extended signal lines are densely disposed, is identical to an off-potential of the scanning lines Vgoff, and the BM potential at the display pixel portion becomes the average of the potentials of the scanning lines, the counter electrode, and the pixel electrodes.
As described above, since the BM potentials differs depending upon the location, an electric current flows between the extended scanning lines and the display pixels through the BM to make the BM potential homogeneous. During such current flow, the BM potential, in a region of a few millimeters from the side where the extended scanning lines are arranged, becomes lower than the potential of the counter electrode. At this time, since a strong transverse electric field is generated from the BM edge to the columns at both edges of a pixel, the liquid crystal will be subjected to the twist-deformation at the columns, so that the area glitters during displaying the black. If the resistance of the BM is sufficiently low, BM potential is homogenized almost spontaneously, this local glittering is not observed. However, when the resistance of the BM is high, the phenomenon of the local glittering in the black color is observed at the side where the extended scanning lines are arranged, after the displaying has been operated for a while.
The BM potential of the display portion is lower than that of the counter electrode due to the influence of the scanning lines which are subjected to a large negative bias. Such a potential difference between the BM and the counter electrode causes migration of electric charge from the BM to the pixel display region. Such migration of electric charge is remarkable when the resistance of BM is lower. Therefore, the problem arises that the BM with a low resistance is likely to cause after images of short duration.
The objects of the present invention are to provide an active matrix-type liquid crystal display device which is capable of maintaining the BM potential at an appropriate value, while maintaining the BM in a floating condition, and obtaining an excellent image quality which does not include after image or unevenness.